(a) Field of the Invention
The present invention relates to a method and system for diagnosing a state of a fuel cell, and more particularly, to a method and system for diagnosing a state of a fuel cell to improve the reliability of diagnosis by calculating a signal to noise ratio.
(b) Description of the Related Art
Generally, a fuel cell includes an electrode that provokes an electrochemical reaction, a polymer electrolyte membrane that transfers hydrogen ions generated by the reaction and a separator that supports the electrode and the polymer electrolyte membrane. The polymer electrolyte fuel cell (hereinafter, referred to a fuel cell) has merits in that the fuel cell is high in efficiency compared with other shapes of fuel cells, great in the current density and the output density, the ignition time is short, and at the same time, it is not corroded nor required for the regulation of electrolyte since the polymer electrolyte fuel cell uses solid electrolyte. In addition, since the fuel cell is an environmentally friendly power source that produces no exhaust except only pure water, currently research is being conducted in automobile industries.
Commonly, the fuel cell life span and performance are influenced by the driving condition of the fuel cell. Accordingly, in the related industry, the diagnosis of fuel cell state is performed to obtain the information for maintaining the driving condition of the fuel cell in the optimal condition. Generally, the method for diagnosing the state of fuel cell includes a method of measuring the current-voltage curve and a method of measuring the impedance of the fuel cell. The measurement of fuel cell impedance is commonly measured by flowing alternating current (AC) current through the fuel cell or calculating the impedance of the fuel cell by outputting AC current from the fuel cell and measuring AC voltage of the fuel cell. In particular, the following relation is established generally.
                                                        V              ac                        ⁡                          (              f              )                                                          I              ac                        ⁡                          (              f              )                                      =                  Z          ⁡                      (            f            )                                              Equation        ⁢                                  ⁢        1            
wherein, Vac(f) represents the AC voltage of the fuel cell at frequency f, Iac(f) represents the AC current of the fuel cell at frequency f, and Z(f) represents the impedance of the fuel cell at frequency f. Meanwhile, it is common that the voltage of the fuel cell changes significantly by acceleration and deceleration of the fuel cell vehicle. The voltage spectrum of the fuel cell spreads to a wide range. In addition, owing to the operation frequencies of the motor, the inverter and the converter mounted on the fuel cell vehicle, the voltage of the fuel cell has the spectrum of very diverse and irregular.
Accordingly, when the spectrum of the fuel cell voltage and the frequency range for diagnosing the fuel cell state are overlapped, the reliability of diagnosis is degraded by increasing the measurement error of the diagnosis of the fuel cell. In the fuel cell vehicle environment, an effort for improving the accuracy of the fuel cell impedance measurement has been conducted since the performance, the durability and the usability of the fuel cell is related to the fuel cell state. However, more reliable measurement technique of the fuel cell impedance is needed since the conventional measurement technique of impedance has a problem of degrading reliability due to the big measurement error of diagnosing the fuel cell state due to the problems described above.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.